In ethylene polymerization processes carried out in continuous, particularly in those carried out in gas phase, there is the need to face up to the formation of polymer agglomerates, such as polymer sheeting or chunks, in the polymerization reactor. Polymer agglomerates involve many negative effects: for example, they can disrupt the discharge of polymer from the reactor by plugging the polymer discharge valves. Furthermore, the agglomerates may also partially cover the fluidization grid of the reactor with a loss in the fluidization efficiency.
The presence of fine polymer particles in the polymerization medium favors the formation of polymer agglomerates (so-called fines). Those fines may be present as a result of introducing fine catalyst particles or as a result of the breakage of catalyst and/or polymer particles within the polymerization reactor. The fines are believed to deposit onto and electrostatically adhere to the inner walls of the polymerization reactor and on the equipment for recycling the gaseous stream such as, for example, the heat exchanger. If the fines remain active, then the particles will grow in size resulting in the formation of agglomerates, also caused by the partial melting of the polymer itself. Those agglomerates, when formed within the polymerization reactor, tend to be in the form of sheets or little chunks. Agglomerates can also partially plug the heat exchanger designed to remove the heat of polymerization reaction.
Several solutions have been proposed to solve the problem of formation of agglomerates during a gas-phase polymerization process. Such solutions include the deactivation of the fine polymer particles, the control of the catalyst activity and, above all, the reduction of the electrostatic charge by introducing antistatic agents inside the reactor.
EP 359444 describes the introduction into the polymerization reactor of small amounts of an activity retarder in order to keep substantially constant either the polymerization rate or the content of transition metal in the polymer produced. The process is said to produce a polymer without forming agglomerates.
U.S. Pat. No. 4,803,251 describes a process for reducing the polymer sheeting by utilizing a group of chemical additives, which generate both positive and negative charges in the reactor, and which are fed to the reactor in an amount of from about 0.1 to about 25 ppm based on the monomer feed, preferably ethylene, in order to prevent the formation of undesired positive or negative charges.
EP 560035 describes a polymerization process in which an anti-fouling compound is used to eliminate or reduce the build-up of polymer particles on the walls of a gas-phase polymerization reactor. This anti-fouling compound is preferably selected from alkydiethanolamines, which may be fed at any stage of the gas-phase polymerization process in an amount greater than 100 ppm by weight with respect to the produced (co)polymer. Said anti-fouling compound, when used in ethylene/propylene copolymerization, is capable to selectively inhibit the polymerization on polymer particles smaller than 850 μm that are responsible for fouling problems and polymer sheeting.
WO2003/033543 describes a process for the gas-phase (co) polymerization of olefins in a fluidized bed reactor wherein fouling is prevented and/or flowability of polymer is improved thanks to the use of a specific class of process aid additives.
WO2010/144080 describes catalyst systems for the polymerization of olefins containing a solid titanium catalyst component and an antistatic agent.
Despite the progresses made by the solutions proposed hitherto, it would still be desirable to further reduce the formation of so-called fines, particularly in gas-phase polymerization processes, without impairing the other features of the process.